(1) Field of the Invention
The present invention relates to an improved cooling microcircuit for use in an airfoil portion of a turbine engine component.
(2) Prior Art
In a gas turbine engine, the turbine airfoils are exposed to temperatures well above their material limits. Industry practice uses air from the compressor section of the engine to cool the airfoil material. This cooling air is fed through the root of the airfoil into a series of internal cavities or channels that flow radially from root to tip. The coolant is then injected into the hot mainstream flow through film-cooling holes. Typically, the secondary flows of a gas turbine blade are driven by the pressure difference between the flow source and the flow exit under high rotational forces. The turbine blades rotate about an axis of rotation 11. As shown in FIG. 1, to increase the convective efficiency of the cooling system in the blade, a series of cooling microcircuits 10 are placed inside the walls 12 and 14 of the airfoil portion 16. Each of the cooling microcircuits 10 has a plurality of outlets or slots 15 for allowing a film of cooling fluid to flow over external surfaces of the airfoil portion 16.
As the coolant inside each cooling microcircuit 10 heats up, the coolant temperature increases; thus, increasing the microcircuit convective efficiency. The other form of cooling which may be required for this type of turbine airfoil is film cooling as the cooling air discharges into the mainstream through a microcircuit slot 15.
FIG. 2 illustrates a cooling microcircuit configuration 18 which may be incorporated into one or more of the walls 12 and 14, typically the pressure side wall 12. The configuration 18 has three inlets 20 for introducing a cooling fluid into the microcircuit, a microcircuit pedestal bank 21, and two slot exits 22. The shape of the pedestals 24 was conceived so that a minimum metering area may be provided for the coolant flow before it enters each of the slots 22. Initially, the symmetry of each of the last pedestals 24 seems to indicate uniform flow and flow re-distribution to fill the slot exit 22. However, one of the cooling fluid jets 23, as shown in FIG. 3, tends to overpower one 25 of the other exit jets. As a result of the jet unbalance, the film exiting the cooling microcircuit slots 22 is uneven. The resulting film protection is decreased, substantially leading to entrapment of hot gases in the side of the lower momentum jet.